1. Field of the Invention
The present invention relates to an image-forming apparatus which comprises electron-emitting devices and image-forming members for forming an image upon irradiation of electron beams, and more particularly to an image-forming apparatus which employs fluorescent substances as the image-forming members.
2. Related Background Art
Heretofore, two types of electron-emitting devices are known; i.e., a thermionic cathode device and a cold cathode device. Known cold cathode devices include electron-emitting devices of surface conduction type, field emission type (hereinafter abbreviated to FE), metal/insulating layer/metal type (hereinafter abbreviated to MIM), etc.
One example described in, e.g., M. I. Elinson, Radio Eng. Electron Phys., 10, 1290, (1965) and other later-described examples are known as surface conduction electron-emitting devices.
A surface conduction electron-emitting device utilizes a phenomenon that when a thin film having a small area is formed on a substrate and a current is supplied to flow parallel to the film surface, electrons are emitted therefrom. As to such a surface conduction electron-emitting device, there have been reported, for example, one using a thin film of SnO.sub.2 by Elinson cited above, one using an Au thin film [G. Dittmer: "Thin Solid Films", 9, 317 (1972)], one using a thin film of In.sub.2 O.sub.3 /SnO.sub.2 [M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975)], and one using a carbon thin film [Hisashi Araki et al.: "Vacuum", Vol. 26, No. 1, 22 (1983)].
As a typical configuration of those surface conduction electron-emitting devices, FIG. 30 shows a plan of the device proposed by M. Hartwell, et al. in the above-cited paper. In FIG. 30, denoted by reference numeral 301 is a substrate and 304 is a conductive thin film made of a metal oxide formed by sputtering. As shown, the conductive thin film 304 is formed into an H-shaped pattern in a plan view. The conductive thin film 304 is subjected to an energizing process called forming by energization (described later) to form an electron-emitting region 305. The dimensions indicated by L and W in the drawing are set to 0.5-1 mm and 0.1 mm, respectively. Although the electron-emitting region 305 is shown as being rectangular centrally of the conductive thin film 304, the region 305 is illustrated so only for the convenience of drawing and does not exactly represent the actual position and shape thereof.
In those surface conduction electron-emitting devices, including the one proposed by M. Hartwell et al., it has heretofore been customary that, before starting emission of electrons, the conductive thin film 304 is subjected to an energizing process called forming by energization to form the electron-emitting region 305. The term "forming by energization" means a process of applying a DC voltage being constant or rising very slowly at a rate of, for example, 1 V/minute, across the conductive thin film 304 to locally destroy, deform or denature it to thereby form the electron-emitting region 305 which has been transformed into an electrically high-resistance state. This produces a fissure in a portion of the conductive thin film 304 which has been locally destroyed, deformed or denatured. When an appropriate voltage is applied to the conductive thin film 304 after the forming by energization, electrons are emitted from the vicinity of the fissure.
Examples of FE electron-emitting devices are described in, e.g., W. P. Dyke & W. W. Dolan, "Field emission", Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, "Physical properties of thin-film field emission cathodes with molybdenium cones", J. Appl. Phys., 47, 5248 (1976).
As one typical configuration of the FE devices, FIG. 31 shows a section of the device proposed by C. A. Spindt. In FIG. 31, denoted by reference numeral 310 is a substrate, 311 is an emitter wiring made of any suitable conductive material, 312 is an emitter cone, 313 is an insulating layer, and 314 is a gate electrode. When an appropriate voltage is applied between the emitter cone 312 and the gate electrode 314, this device emits electrons from the tip end of the emitter cone 312.
Other than the laminated structure shown in FIG. 31, there is also known another configuration of the FE devices in which an emitter and a gate electrode are arranged side by side on a substrate substantially parallel in the substrate plane.
One example of MIM electron-emitting devices is described in, e.g., C. A. Mead, "Operation of tunnel-emission devices", J. Appl. Phys., 32, 646 (1961). One typical configuration of the MIM devices is shown in a sectional view of FIG. 32. In FIG. 32, denoted by reference numeral 320 is a substrate, 321 is a lower electrode made of metal, 322 is a thin insulating layer being about 100 angstroms thick, and 323 is an upper electrode made of metal and being about 80-300 angstroms thick. When an appropriate voltage is applied between the upper electrode 323 and the lower electrode 321, this MIM device emits electrons from the surface of the upper electrode 323.
The above-described cold cathode devices can emit electrons at a lower temperature than needed in thermionic cathode devices, and hence require no heaters for heating the devices. Accordingly, the cold cathode devices are simpler in structure and can be formed in a finer pattern than thermionic cathode devices. Further, even when a number of cold cathode devices are arrayed on a substrate at a high density, the problem of hot-melting the substrate is less likely to occur. Additionally, unlike thermionic cathode devices which have a low response speed because they operate under heating by heaters, the cold cathode devices are also advantageous in having a high response speed.
For that reason, intensive studies have been focused on applications of the cold cathode devices.
Of the cold cathode devices, particularly, the surface conduction electron-emitting device is simple in structure and easy to manufacture, and hence has an advantage that a number of devices can be formed into an array having a large area. Therefore, methods of arraying a number of devices and driving them have been studied as disclosed in, e.g., Japanese Patent Application Laid-Open No. 64-31332 in the name of the same assignee.
Various applications of surface conduction electron-emitting devices have also been studied in the fields of image-forming apparatus such as image display devices and image recording devices, charged beam sources, and so on.
As an application to image display devices, particularly, one employing a combination of a surface conduction electron-emitting device and a fluorescent substance which emits light upon irradiation of an electron beam has been researched as disclosed in, e.g., U.S. Pat. No. 5,066,883 issued to the same assignee and Japanese Patent Application Laid-Open No. 2-257551 and No. 4-28137 both in the name of the same assignee. Such an image display device employing the combination of a surface conduction electron-emitting device and a fluorescent substance is expected to have superior characteristics to other conventional image display devices. As compared with display devices using liquid crystals which have recently become popular, for example, the above combined display device is superior in that it does not require any backlight because of being self-luminous and has a wider field angle of vision.
One of methods of arraying a number of FE devices and driving them is disclosed in, e.g., U.S. Pat. No. 4,904,895 issued to the same assignee. As an application example of FE devices to an image display device, there is known a flat display device reported by R. Meyer, for example. [R. Meyer: "Recent Development on Microtips Display at LETI", Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6-9 (1991)].
One example in which an array of numerous MIM devices is applied to an image display device is disclosed in, e.g., Japanese Patent Application Laid-Open No. 3-55738 in the name of the same assignee.
The inventors have attempted manufacture of cold cathode devices by using a variety of materials, methods and structures, including the ones described above as the prior art. Also, the inventors have studied a multi-electron beam source having an array of numerous cold cathode devices, and an image display device in which the multi-electron beam source is employed.
For example, the inventors have tried a multi-electron beam source using an electrical wiring method as shown in FIG. 28. Specifically, the multi-electron beam source is arranged such that a number of cold cathode devices are arrayed two-dimensionally and wired into a matrix pattern as shown.
In FIG. 28, denoted by 401 is a cold cathode device symbolically shown, 402 is a row-directional wiring, and 403 is a column-directional wiring. While the row- and column-directional wirings 402, 403 have in fact finite electric resistances, these resistances are indicated as wiring resistors 404, 405 in the drawing. The illustrated wiring arrangement will be referred to as a simple matrix wiring.
FIG. 28 shows the array of a 6.times.6 matrix for the convenience of drawing. However, the matrix size is not of course limited to the illustrated one. A multi-electron beam source for an image display device, for example, is formed by arraying and wiring cold cathode devices in number enough to provide desired image display.
In a multi-electron beam source having cold cathode devices arrayed with the simple matrix wiring, appropriate electric signals are applied to the row-directional wirings 402 and the column-directional wirings 403 for emitting desired electron beams. To drive any one row of cold cathode devices in the matrix, for example, a select voltage Vs is applied to the row-directional wiring 402 to be selected and, simultaneously, a non-select voltage Vns is applied to the other row-directional wirings 402 not selected. In synch with application of the voltages to the row-directional wirings 402, a drive voltage Ve for enabling the devices to emit electron beams is applied to the column-directional wirings 403. With this method, ignoring voltage drops through the wiring resistances 404 and 405, the voltage Ve-Vs is applied to the cold cathode devices in the selected row and the voltage, Ve-Vns is applied to the cold cathode devices in the non-selected rows. If the voltages Ve, Vs and Vns are set to have suitable values, electron beams are emitted with the desired intensity only from the cold cathode devices in the selected row. Also, if the drive voltage Ve applied to the column-directional wirings 403 is set to have respective different values, electron beams are emitted with the different intensities from the individual cold cathode devices in the selected row. Further, if the duration in which the drive voltage Ve is applied is changed, the period of time in which the electron beam is emitted can also be changed.
Accordingly, the multi-electron beam source having cold cathode devices arrayed with the simple matrix wiring is applicable to various fields. For example, that multi-electron beam source can be suitably used as an electron source for an image display device by properly applying electric signals to the cold cathode devices in accordance with image information.
However, the following problems have been raised with practical use of the multi-electron beam source having cold cathode devices arrayed with the simple matrix wiring.
FIG. 29 shows, by way of example, a section of one prior art image display panel including cold cathode devices and fluorescent substances. In FIG. 29, denoted by 410 is a back plate, 411 is a cold cathode device formed on the back plate, 412 is a side wall, 413 is a face plate, and 414 is a fluorescent substance disposed on an inner surface of the face plate. A vacuum container is formed by the back plate 410, the side walls 412 and the face plate 413. In such a display panel, an image is displayed by irradiating an electron beam e.sup.- emitted from the cold cathode device 411 to the fluorescent substance 414, causing the fluorescent substance 414 to radiate visible light VL.
However, the above display panel accompanies the problem of rendering a displayed image deficient or uneven in luminance or giving rise to unexact coloration because of insufficient accuracy of assembly during manufacture.
More specifically, when assembling the vacuum container, the components are firmly bonded to each other by using an adhesive, such as frit glass, to establish and maintain air tightness, but a high temperature not lower than 400.degree. C. is required to melt the frit glass. Even if the components are positioned with sufficiently high accuracy beforehand, their positions are apt to deviate in the heating step due to thermal expansion of the components themselves and fixing jigs and, once bonded, it is practically impossible to correct resultant deviations of the positions.
Accordingly, there often occurs an uncorrectable positional deviation between the back plate 410 including the cold cathode devices formed thereon and the face plate 414 including the fluorescent substances disposed thereon.
As an alternative, even if the panel structure is modified such that the back plate 410 including the cold cathode devices formed thereon and the face plate 414 including the fluorescent substances disposed thereon are fixed inside a separate vacuum container, the positional relationship between the plates is also apt to deviate similarly due to thermal expansion in a heating step needed to seal off the separate vacuum container. Further, once the vacuum container is sealed off, it is practically impossible in the modified panel structure to correct the positions of both the plates fixed inside the container.
If the positional relationship between the cold cathode devices and the fluorescent substances is deviated, the electron beams e.sup.- emitted from the cold cathode devices do not precisely irradiate the corresponding fluorescent substances, resulting in the problem that image quality is remarkably deteriorated because of lack of an edge of the displayed image, deficiency or unevenness in luminance of the image, or occurrence of inexact coloration. Moreover, since the direction and magnitude of the positional deviation are varied for each of display panels, it is very difficult to provide a number of display panels which have uniform display capabilities.